Minor: add / fix comments (related to KT-20752)

compiler/frontend/src/org/jetbrains/kotlin/resolve/calls/smartcasts/DelegatingDataFlowInfo.kt

@@ -97,6 +97,8 @@ internal class DelegatingDataFlowInfo private constructor(
             languageVersionSettings: LanguageVersionSettings,
             typeInfo: SetMultimap<DataFlowValue, KotlinType>? = null,
             affectReceiver: Boolean = true,
+            // TODO: remove me in version 1.3! I'm very dirty hack!
+            // In normal circumstances this should be always true
             recordUnstable: Boolean = true
     ): Boolean {
         if (value.isStable || recordUnstable) {

compiler/testData/diagnostics/tests/smartCasts/castchecks/impossible.kt

@@ -14,12 +14,12 @@ fun StringList.remove(s: String?) = s ?: ""
 fun foo(list: StringList, arg: Unstable) {
     list.remove(arg.first)
     if (arg.first <!USELESS_CAST!>as? String<!> != null) {
-        // Ideally should have smart cast impossible here
+        // Should be still resolved to extension, without smart cast or smart cast impossible
         list.remove(arg.first)
     }
     val s = arg.first <!USELESS_CAST!>as? String<!>
     if (s != null) {
-        // Ideally should have smart cast impossible here
+        // Should be still resolved to extension, without smart cast or smart cast impossible
         list.remove(arg.first)
     }
 }

compiler/testData/diagnostics/tests/smartCasts/elvis/impossible.kt

@@ -16,7 +16,7 @@ fun String.isEmpty() = this == ""
 fun foo(list: StringList, arg: Unstable) {
     list.remove(arg.first)
     if (arg.first?.isEmpty() ?: false) {
-        // Ideally should have smart cast impossible here
+        // Should be still resolved to extension, without smart cast or smart cast impossible
         list.remove(<!SMARTCAST_IMPOSSIBLE!>arg.first<!>)
     }
 }
@@ -34,7 +34,7 @@ fun BooleanList.remove(b: Boolean?) = b ?: false
 fun bar(list: BooleanList, arg: UnstableBoolean) {
     list.remove(arg.first)
     if (arg.first ?: false) {
-        // Ideally should have smart cast impossible here
+        // Should be still resolved to extension, without smart cast or smart cast impossible
         list.remove(<!SMARTCAST_IMPOSSIBLE!>arg.first<!>)
     }
 }